1. Field of the Invention
The present invention relates to a curing component for epoxy resins, a process for its preparation and the use thereof in curable mixtures.
2. Description of Related Art
It is known that alcohols, in particular phenol or alkylated phenols, accelerate the curing reaction between oxiranes and amino groups (cf. U.S. Pat. No. 3,366,600, EP-B 0 114 875, DE-C 10 43 629 and DE-C 20 25 343). It has proved advantageous in these case to form a single compound that combines the phenol component together with the amine component. In DE-C 15 43 754, a hydroxyaryl-substituted aliphatic carboxylic acid is linked with a polyamine via an amide bond to produce a single compound having both a phenol component and an amine component. In this process, however, one amino group of the polyamine, which is required for the amide bond, is lost for the curing operation. In DE-A 32 33 565 and EP-A 0 114 875, the reaction product of a Mannich reaction of aldehydes and polyamines with phenol or alkylphenols, for example cresols, p-tert-butylphenol or nonylphenol, is therefore proposed as the curing component, and in EP-A 0 066 447 reaction products with arylphenols, for example benzylphenol, also are proposed.
U.S. Pat. No. 2,974,121 also discloses adducts of phenols with polyamines which are used for curing epoxy resins, and in particular are used as coatings. The suitable polyamines have at least one, but not more than two amino groups per molecule, the amino groups in each case being bonded to an aliphatic carbon atom. Epoxide-amine adducts are preferred in this document. The phenols disclosed in this document are those that have in each case at least one monohydroxyphenyl group per molecule. Phenols which contain exclusively polyhydroxyphenyl groups, such as resorcinol, hydroquinone and phloroglucinol, are said to provide coatings with matt surfaces, and are therefore not useful for the present invention. The adducts disclosed in this document are formed by heating, without the addition of catalysts, to temperatures between 4 and 104.degree. C., preferably up to 52.degree. C. The adducts are salts of the acid phenol with the amine; the salt is present in an equilibrium, the phenol being liberated again during the (amine-consuming) curing reaction.
These combinations have all proved to be good, but also have disadvantages, such as, in some cases, a lack of acceleration at low temperatures (.ltoreq.10.degree.0C.) and under high atmospheric humidity, and a lack of resistance to dilute organic acids, such as, for example, acetic acid. Since all these aforementioned combinations comprise phenol or alkylated phenols, they can furthermore be employed to only a limited extent from a toxicological aspect due to the risk to the user during application. When contacted with foodstuffs, the phenols are readily extracted from the coating due to the aqueous dilute organic acids contained therein, such as acetic acid, tartaric acid or citric acid, and thus spoil the foodstuff.
DE-C 10 43 629 also mentions, in addition to the customary phenolic components, polyhydric phenols as possible reactants for curing components which can be prepared by a Mannich reaction. Compounds such as resorcinol and hydroquinone, however, produce highly crosslinked Mannich bases of undesirably high viscosity because of their high reactivity. The increased polarity of polyhydric phenols compared with monohydric phenolic components furthermore causes greater blushing of the coatings produced from them, for example during absorption of water (action of moisture).